Continuous cold feed, high accuracy preform production with Combex. (Process Machinery).

Conventional preform (blank) production relies on a ram-type, hydraulically activated extruder, fitted with a cutter using a fixed speed and an accurate ram displacement control. Production is thus dependent on a batch-off, discontinuous process requiring a dedicated upstream mill to provide the machine with preheated/milled feed material. The conventional process is also fairly labor intensive, since the ram must be refilled every time a batch is produced.

New extruder/gear pump technology provides for a vast improvement over the conventional method of blank production outlined above. Combex preform production systems rely on just such technology (figure 1). A cold feed strip is fed into the feed opening of the extruder component of the system, which plasticizes, preheats and conveys the compound to the gear pump entry. The gear pump, in turn, forces the rubber through the die opening, where it is cut to the required size by a rotary cutting knife. Gear pump technology is critical for the success of the system, since it allows for excellent pumping (pressure inducing) efficiency and exceptional throughput accuracy. Notably, gear pump rotors and bearings are proving reliable, with typical replacement spans measured in years and not months.

[FIGURE 1 OMITTED]

Control of the system is by a purpose developed PLC. Pressure sensors are fitted at gear pump entry and gear pump exit. Gear pump entry pressure must be maintained at a specific pressure level to ensure effective filling of the gear teeth. The extruder speed is therefore regulated by the pump entry pressure so that the pump is consistently supplied with material at the required pressure.

Since the gear pump is a positive displacement pump, throughput is linearly (and accurately) dependent on gear pump rotational speed. In automatic mode, the cutting knife rotates at a fixed speed. The PLC control then automatically adjusts the gear pump speed according to the entered blank weight. This is accomplished via a calculation using the specific gravity of the compound and the specific throughput of the gear pump, so that the gear pump feeds the cutting knife at the exact rate required to achieve the cutting to weight. A new development enables the operator to simply enter the desired weight of the required preform into the touch screen operating panel. An initial sample produced must consequently be weighed and the result entered on the touch screen, after which Combex automatically self adjusts and produces a continuous stream of high precision preforms. Specific gravity data are thus no longer required.

For bigger production pieces, Combex can be operated in stop-start or cutter mode. An encoder on the gear pump signals the cutting knife when to cut, so that the required blank piece is precisely produced.

The Combex system offers clear and proven advantages over the conventional systems, as outlined below.

* Continuous process: Combex can be fed continuously by strip material, and produces a continuous, uninterrupted supply of high precision blanks. The labor requiring task of refilling the hydraulic ram is therefore made redundant.

* Cold feed: Combex can be cold fed with strip material, so that an in-line mill is no longer required, thereby saving operating, maintenance and machine investment costs. Blank production can also be relocated from the mill room to right next to the presses, with obvious workflow improvements.

* Minimal floorspace requirement: In addition to no longer requiring production space for a dedicated in-line mill, the Combex machine is by itself of very compact design. A Combex 90, suitable for up to about 660 lbs./hr. production (depending on compound), has a frame footprint of 7.5 ft. x 3.3 ft. The Combex 45, the smallest of the Combex family capable of about 130 lbs./hr. production, has a frame footprint of just 3.2 ft x 2.4 ft.

* No air entrapment: The high pump inlet pressure prevents air inclusions from reaching the gear teeth, so that air does not reach the die and thus is not contained in the preform produced. Conversely, a ram type of machine actively creates air inclusions, and despite efforts to counter this by means of a vacuum pump, air inclusions are inherent to ram type preform production.

* Consistent, high accuracy: An integrated cutting speed/gear pump speed control results in very accurate production. Typical blank weight accuracy is [+ or -] 0.5% by weight, even for very small blanks. The best accuracy that today's ram type machines can produce is usually about [+ or -] 1%. Furthermore, for the Combex system, this accuracy is continuous. In contrast, ram systems typically produce scrap at the start and at the end of the ram stroke due to inaccuracy.

* All electric: All powered systems on the Combex, including extruder and gear pump, are electrically driven. The system is thus inherently cleaner with no risk of hydraulic oil contaminating the rubber. Maintenance is also much simpler.

* Accurate temperature control: Standard ram systems feature simple heaters for barrel and head, whereas Combex systems feature temperature control units that can heat up as well as cool down the surfaces in contact with rubber. Due to the cooling capability, rapid production changes can be performed when changing from a material which is processed at warmer temperatures to one that is processed at cooler temperatures (such as silicone).

* Process flexibility: By elevating the gear pump inlet pressure above the minimum pressure required to fill the gear pump teeth, additional work may be performed on the compound by the extruder, since the extruder will then speed up in order to compensate for the increase in required pump inlet pressure. The mastication performed on the compound may therefore be changed while keeping the gear pump speed and output constant. A single Combex machine is hence able to process a wide range of materials, including fluoroelastomers, without mechanical modifications being required.

* Easy operation: A simple to operate touch screen panel enables very easy process control and machine operation. Die change is performed within two to three minutes by screwing off the die holder and slotting in a new die. Combex systems are also designed for easy maintenance, with rapid disassembly of cutter and gear pump possible, including easy drawing of the gear pump rotors.

Overall, the above benefits of the Combex system of producing preform blanks results in significantly reduced production costs at a notable product quality increase. A material cost saving is achieved due to no start- and end-of-stroke scrap, as well as constant and improved accuracy. Labor requirements are reduced, since the machine is virtually operator-free once it has started up and is correctly fed with strip compound. Combex also reduces maintenance costs, since the electrically powered extruder gear pump machine does not have any hydraulics to clean and attend to. Furthermore, the deletion of the upstream mill lowers inventory costs and further reduces overall maintenance costs. End product quality should be improved due to fewer air inclusions.

Combex systems are available in various sizes. The smallest size is shown in figure 2.

[FIGURE 2 OMITTED]

Table 1 summarizes the specifications of the various standard sizes available. The same attributes that make Combex so suitable for preform production also make it a useful system for direct, large mold filling and straining.

Table 1 - Combex specifications

Combex type           45                90               120
Throughput*       150 lbs./h.       700 lbs./h.     1,500 lbs./h.
Cutting speed   0-500 cuts/min.   0-70 cuts/min.,    stop/start
Pressure on                        Up to 400 bar
  compound